An alloy made of titanium and columbium has been previously used only for its superconducting properties in the electronic industry, but not as structural or fastener material. The problem was to provide a fastener of light weight, superior comparable shear strength and a readily buckable tail.
Considering the rivet applications, for instance, in 6A1-4V and CP titanium sheets, previously the following disadvantage occurred:
Aluminum rivets cannot be used, although they are easy to upset and are light in weight, because they rapidly corrode in the more noble titanium sheet, and because aluminum rivets cannot be used at temperatures above approximately 350.degree.F, and the strength values of such aluminum rivets are relatively too low.
Monel rivets are used at strength levels of 50 KSI shear, but they are heavy compared to the weight of such sheets and they are harder to upset and distort thin sheets.
CP titanium rivets can be used and they are even lighter, but they cause unacceptable sheet distortion in thin sheets and they are appreciably more difficult to upset thus requiring heavier rivet guns and bucking bars which result in greater operator fatigue, which is a very important factor and are difficult to upset in hard-to-get-at places where bucking bars of less than ideal shape have to be used.
A-286 rivets were tried as they are stronger at room temperature and also at elevated temperature up to 1300.degree.F, however they are much heavier and are very hard to drive and distort thin sheets very badly.
Several so-called Beta alloys of Titanium were tried but the driving was unacceptably hard and such rivets were heavy.
In applications in aluminum sheets such as 2024T3 and 7075T6, heretofore cadmium plated Monel solid rivets were used as well as CP titanium solid rivets which would have satisfactory design shear strength. The major known use of such Monel and CP solid rivets in an aluminum aircraft structure is in the Anglo-French Concorde. In this use CP titanium was substituted for Monel to save weight, but CP titanium increases the corrosion rate of aluminum in salt spray environments and although the CP titanium rivets are installed with wet zinc chromate primer, or other anticorrosion insulating coating, applied to the rivet or hole, it is not accepted as the complete answer because the primer in actual practice either incompletely covers the aluminum surface or is dry at the time the rivet is installed and flakes off. Furthermore, accelerated galvanic corrosion of the aluminum takes place which is apt to cause looseness of the joint. Furthermore the CP titanium may absorb hydrogen from the galvanic action, hydrides may form and the rivet may fail.
So far as one of the methods of cold working is concerned, the closest reference of which applicant is aware is U.S. Pat. No. 3,626,531 issued on Dec. 14, 1971 to M. R. Mazer, et al., which patent, however, is concerned with the rivets made of material heretofore enumerated in previous use, and the steps do not provide for a predetermination of the ductile tail length. In applicant's method substantially the entire grip length of the rivet is work hardened so as to result in the particular ratio between the unworked ductile tail portion and the diameter of the rivet shank.
Applicant's particular rivet is not only light but it has superior shear strength proportioned to the grip length of the rivet and it fulfills a need which no other solid rivet heretofore accomplished. The rivets of the herein application are unique in that they provide the necessary shear strength, yet they are easier to drive than any other rivet of comparable shear strength; the herein rivets may be driven in thin sheets without sheet distortion, maintain their strength and ductility very well at temperatures up to 800.degree.F; their strength and ductility and other physical properties are stable based on tests up to 1000 hours at 800.degree.F; because of the ease of driving, smaller rivet hammers and bucking weights can be used, operator fatigue is reduced, and rivets may be used in hard-to-get-at places with bent or other less than ideal bucking bar shapes, without resulting in lopsided bucked rivet heads. It was found that after prolonged exposure at 800.degree.F the herein rivets actually agehardened and became stronger. For instance, after 244 hours at 800.degree.F, 89% cold worked rivets of this material had a tensile strength of 130 KSI with an elongation value of 24% compared to its initial strength of 86 KSI at 24% elongation. This aging factor in some cases can become very important. The herein rivets made in accordance with the herein method solve the corrosion problem even in the uncoated condition.